Storage gate protection

ABSTRACT

A method of backside illuminated image sensor fabrication includes forming a plurality of photodiodes in a semiconductor material, where the plurality of photodiodes are disposed to receive image light through a backside of the backside illuminated image sensor. The method further includes forming a transfer gate coupled to extract image charge from a photodiode in the plurality of photodiodes, and forming a storage gate coupled to the transfer gate to receive the image charge. Forming the storage gate includes forming an optical shield in the semiconductor material; depositing a gate electrode proximate to a frontside of the semiconductor material; and implanting a storage node in the semiconductor material, where the storage node is disposed in the semiconductor material between the optical shield and the gate electrode.

REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.15/362,391, filed on Nov. 28, 2016, the contents of which areincorporated herein by reference.

TECHNICAL FIELD

This disclosure relates generally to semiconductor fabrication, and inparticular but not exclusively, relates to storage gate construction.

BACKGROUND INFORMATION

Image sensors have become ubiquitous. They are widely used in digitalstill cameras, cellular phones, security cameras, as well as, medical,automobile, and other applications. The technology used to manufactureimage sensors has continued to advance at a great pace. For example, thedemands of higher resolution and lower power consumption have encouragedthe further miniaturization and integration of these devices.

For high-speed image sensors, a global shutter can be used to capturefast-moving objects. A global shutter typically enables all pixel cellsin the image sensor to simultaneously capture the image. For slowermoving objects, the more common rolling shutter is used. A rollingshutter normally captures the image in a sequence. For example, each rowwithin a two-dimensional (“2D”) pixel cell array may be enabledsequentially, such that each pixel cell within a single row captures theimage at the same time, but each row is enabled in a rolling sequence.Accordingly, each row of pixel cells captures the image during adifferent image acquisition window. For slow moving objects the timedifferential between each row generates image distortion. Forfast-moving objects, a rolling shutter causes a perceptible elongationdistortion along the object's axis of movement.

To implement a global shutter, storage structures can be used totemporarily store the image charge acquired by each pixel cell in thearray while it awaits readout from the pixel cell array. Factors thataffect performance in an image sensor pixel cell having a global shutterinclude shutter efficiency, dark current, white pixels and image lag.Moreover pollution of image charge in the storage structure may have adeleterious effect on image quality.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive examples of the invention are describedwith reference to the following figures, wherein like reference numeralsrefer to like parts throughout the various views unless otherwisespecified.

FIG. 1A is a cross sectional illustration of an example backsideilluminated image sensor, in accordance with the teachings of thepresent invention.

FIG. 1B is a cross sectional illustration of an example backsideilluminated image sensor, in accordance with the teachings of thepresent invention.

FIG. 1C is a circuit diagram of the image sensor in FIGS. 1A-1B, inaccordance with the teachings of the present invention.

FIG. 2 is a block diagram illustrating one example of an imaging systemwhich may include the image sensor of FIGS. 1A-1C, in accordance withthe teachings of the present invention.

FIGS. 3A-3C illustrate an example method for forming the image sensor ofFIG. 1A, in accordance with the teachings of the present invention.

FIGS. 4A-4C illustrate an example method for forming the image sensor ofFIG. 1B, in accordance with the teachings of the present invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. Also, common but well-understoodelements that are useful or necessary in a commercially feasibleembodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION

Examples of an apparatus and method for storage gate protection in imagesensors are described herein. In the following description, numerousspecific details are set forth to provide a thorough understanding ofthe examples. One skilled in the relevant art will recognize, however,that the techniques described herein can be practiced without one ormore of the specific details, or with other methods, components,materials, etc. In other instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringcertain aspects.

Reference throughout this specification to “one example” or “oneembodiment” means that a particular feature, structure, orcharacteristic described in connection with the example is included inat least one example of the present invention. Thus, the appearances ofthe phrases “in one example” or “in one embodiment” in various placesthroughout this specification are not necessarily all referring to thesame example. Furthermore, the particular features, structures, orcharacteristics may be combined in any suitable manner in one or moreexamples.

Throughout this specification, several terms of art are used. Theseterms are to take on their ordinary meaning in the art from which theycome, unless specifically defined herein or the context of their usewould clearly suggest otherwise. It should be noted that element namesand symbols may be used interchangeably through this document (e.g., Sivs. silicon); however, both have identical meaning.

FIG. 1A is a cross sectional illustration of an example backsideilluminated image sensor 100A. Image sensor 100A includes semiconductormaterial 101, photodiode 103, transfer gate 111, storage gate 113(including storage node 105 and optical shield 107), output gate 115,and floating diffusion 121. Semiconductor material 101 includes aplurality of photodiodes (see FIG. 2) disposed in semiconductor material101 to receive image light through backside 153 of semiconductormaterial 101. Transfer gate 111 is electrically coupled to photodiode103 in the plurality of photodiodes to extract image charge fromphotodiode 103. Storage gate 113 is electrically coupled to transfergate 111 to receive the image charge from transfer gate 111. Asillustrated, storage gate 113 includes a gate electrode (portion 113raised above semiconductor material 101) disposed proximate to frontside151 of semiconductor material 101, an optical shield 107 disposed insemiconductor material 101, and a storage node 105 disposed between thegate electrode and optical shield 107. Optical shield 107 is opticallyaligned with storage node 105 to prevent the image light incident onbackside illuminated image sensor 100A from reaching storage node 105.In the depicted example, storage node 105 and optical shield 107 are atleast partially laterally coextensive with each other, as well as thegate electrode of storage gate 113.

In the depicted example, optical shield 107 is freestanding insemiconductor material 101, and may include at least one of siliconoxide or germanium. As will be described in greater detail later, thisfree standing optical shield 107 may be formed without any etching stepsby implanting, and subsequently annealing, impurity atoms. This providesa way to entirely reflect or absorb light traveling towards storage node105; an otherwise difficult task in a backside illuminated image sensor.One skilled in the art will appreciate that the low index of refractionof silicon oxide may be used to reflect certain wavelengths and indicesof light, while germanium (or GeSi in the case of a siliconsemiconductor material 101) may absorb light traveling towards storagenode 105.

Also depicted is photodiode 103 extending into semiconductor material101 from frontside 151 a certain depth. As shown, in some examples, atleast part of photodiode 103 is disposed proximate to backside 153 ofsemiconductor material 101 and is laterally coextensive with transfergate 111. In other words, a portion of photodiode 103 and transfer gate111 overlap in a vertical direction. Moreover the width of photodiode103 proximate to frontside 151 is smaller than the width of photodiode103 proximate to backside 153. As shown, the wider portion of photodiode103 may be laterally coextensive with the active region of transfer gate111 and the gate electrode of transfer gate 111. In other words, atleast part of the photodiode 103 is disposed proximate to backside 153and is optically aligned with an active region of transfer gate 111.Photodiode 103 absorbs at least some of the image light before the imagelight reaches the active region of transfer gate 111. In some examples,this may be useful since photodiode 103 may absorb some of the lightthat otherwise could be absorbed by the active region of transfer gate111 and degrade image signal quality.

FIG. 1B is a cross sectional illustration of an example backsideilluminated image sensor 100B. Backside illuminated image sensor 100B issimilar in many respects to image sensor 100A in FIG. 1A; however,backside illuminated image sensor 100B has a substantially “T” shapedoptical shield 107 that, as will be explained later, is fabricated in adifferent way than optical shield 107 in FIG. 1A. As illustrated,optical shield 107 extends from backside 153 into semiconductor material101, and a portion of optical shield 107 closest to frontside 151 iswider than a portion of optical shield 107 closest to backside 153. Inother words, the top of the “T” shaped structure is laterally wider thanthe column section of the “T”. In the depicted example, optical shield107 may include at least one of an oxide or a metal. More specifically,optical shield 107 may be lined with silicon oxide and may be filledwith a metal, high-K oxide, semiconductor, or any combination of theaforementioned.

FIG. 1C is an example circuit diagram of the image sensor in FIGS.1A-1B. Pixel cell 100C has a global shutter in accordance with theteachings of the present invention. As shown, pixel cell 100 includesglobal shutter gate transistor 143, photodiode 103, transfer gate 111,storage gate 113 (including optical shield 107), output gate 115,floating diffusion 121, reset transistor 123, amplifier transistor 131,and row select transistor 133. In one example, the amplifier transistor116 is implemented with a source follower coupled transistor. As shownglobal shutter gate transistor 143 is coupled between a V_(GS) voltageand the photodiode 103.

In operation, global shutter gate transistor 143 is coupled toselectively deplete the image charge that has accumulated in photodiode103 by selectively coupling photodiode 103 to voltage V_(GS) in responseto a global shutter signal. Photodiode 103 is disposed in semiconductormaterial 101 of pixel cell 100 to accumulate image charge in response toincident light directed to photodiode 103. Photodiode 103 is coupled totransfer gate 111 to transfer image charge accumulated in photodiode 103to an input of the storage gate 113. Transfer gate 111 allows charge toflow from photodiode 103 into storage gate 113, in response to atransfer signal applied to the gate electrode of transfer gate 111.

In the depicted example, the storage gate 113 is illustrated as beingoptically isolated from image light by optical shield 107. As discussedabove in connection with FIGS. 1A-1B, optical shield 107 prevents lightincident on the backside of the image sensor from entering the storagenode in storage gate 113. This prevents unwanted hole-electron pairsfrom forming in the storage node, and distorting image charge.

The example in FIG. 1C also illustrates that output gate 115 is coupledto an output of storage gate 113 to selectively transfer the imagecharge from the storage gate 113 to floating diffusion 121. Resettransistor 123 is coupled between a reset voltage V_(RESET) and floatingdiffusion 121 to selectively reset the charge in floating diffusion 121in response to a reset signal RST. Amplifier transistor 131 includes anamplifier gate coupled to floating diffusion 121 to amplify the signalon floating diffusion 121 to output image data from pixel cell 100C. Rowselect transistor 133 is coupled between the bitline and amplifiertransistor 131 to output the image data.

FIG. 2 is a block diagram illustrating one example of imaging system 200which may include the image sensors of FIGS. 1A-1C. Imaging system 200includes pixel array 205, control circuitry 221, readout circuitry 211,and function logic 215. In one example, pixel array 205 is atwo-dimensional (2D) array of photodiodes, or image sensor pixels (e.g.,pixels P1, P2 . . . , Pn). As illustrated, photodiodes are arranged intorows (e.g., rows R1 to Ry) and columns (e.g., column C1 to Cx) toacquire image data of a person, place, object, etc., which can then beused to render a 2D image of the person, place, object, etc. However,photodiodes do not have to be arranged into rows and columns and maytake other configurations.

In one example, after each image sensor photodiode/pixel in pixel array205 has acquired its image data or image charge, the image data isreadout by readout circuitry 211 and then transferred to function logic215. In various examples, readout circuitry 211 may includeamplification circuitry, analog-to-digital (ADC) conversion circuitry,or otherwise. Function logic 215 may simply store the image data or evenmanipulate the image data by applying post image effects (e.g., crop,rotate, remove red eye, adjust brightness, adjust contrast, orotherwise). In one example, readout circuitry 211 may readout a row ofimage data at a time along readout column lines (illustrated) or mayreadout the image data using a variety of other techniques (notillustrated), such as a serial readout or a full parallel readout of allpixels simultaneously.

In one example, control circuitry 221 is coupled to pixel array 205 tocontrol operation of the plurality of photodiodes in pixel array 205.For example, control circuitry 221 may generate a shutter signal forcontrolling image acquisition. In the depicted example, the shuttersignal is a global shutter signal for simultaneously enabling all pixelswithin pixel array 205 to simultaneously capture their respective imagedata during a single acquisition window. In another example, imageacquisition is synchronized with lighting effects such as a flash.

In one example, imaging system 200 may be included in a digital camera,cell phone, laptop computer, automobile or the like. Additionally,imaging system 200 may be coupled to other pieces of hardware such as aprocessor (general purpose or otherwise), memory elements, output (USBport, wireless transmitter, HDMI port, etc.), lighting/flash, electricalinput (keyboard, touch display, track pad, mouse, microphone, etc.),and/or display. Other pieces of hardware may deliver instructions toimaging system 200, extract image data from imaging system 200, ormanipulate image data supplied by imaging system 200.

FIGS. 3A-3C illustrate an example method 300 for forming the imagesensor of FIG. 1A. The order in which some or all illustrations appearin method 300 should not be deemed limiting. Rather, one of ordinaryskill in the art having the benefit of the present disclosure willunderstand that some of method 300 may be executed in a variety oforders not illustrated, or even in parallel. Furthermore, method 300 mayomit certain illustrations in order to avoid obscuring certain aspects.Alternatively, method 300 may include additional illustrations that maynot be necessary in some embodiments/examples of the disclosure.

FIG. 3A illustrates providing a plurality of backside illuminatedphotodiodes 303 in semiconductor material 301 along with floatingdiffusion 321. To form optical shield 307, an impurity element isimplanted between backside 353 and a location of the storage node(depicted as an empty dashed-line box since, in the illustrated example,no storage node is formed yet). In one example, the impurity element maybe oxygen, germanium, or any other suitable element or combination ofelements. Although in the depicted example, the impurity is implantedthrough frontside 351, in other examples, the impurity element may beimplanted through backside 353.

FIG. 3B depicts annealing the implanted elements to from a chemicalcompound with the semiconductor material 301. In one example, thesemiconductor material 301 includes silicon and the implanted impurityelements form either SiO₂ or SiGe. Both of these chemical compositionsof optical shield 307 may help block light traveling to storage node305. For example, the low index of refraction of silicon dioxide mayreflect light traveling towards storage node 305, conversely the narrowbandgap of SiGe may allow for absorption of a broad range of wavelengthsof light, including infra-red light and the like.

FIG. 3C illustrates forming additional image sensor device architecture.For example, transfer gates 311 are formed and they couple to extractimage charge from photodiode 303 in the plurality of photodiodes. Othercomponents of storage gate 313 are formed, such that storage gate 313couples to transfer gate 311 to receive the image charge. For example,the gate electrode is deposited proximate to frontside 351 ofsemiconductor material 301, and storage node 305 is implanted insemiconductor material 301 (e.g., with ion beam implantation or thelike). Storage node 305 is disposed in semiconductor material 301between optical shield 307 and the gate electrode.

FIGS. 4A-4C illustrate an example method 400 for forming the imagesensor of FIG. 1B. The order in which some or all illustrations appearin method 400 should not be deemed limiting. Rather, one of ordinaryskill in the art having the benefit of the present disclosure willunderstand that some of method 400 may be executed in a variety oforders not illustrated, or even in parallel. Furthermore, method 400 mayomit certain illustrations in order to avoid obscuring certain aspects.Alternatively, method 400 may include additional illustrations that maynot be necessary in some embodiments/examples of the disclosure.

FIG. 4A illustrates implanting first dopant though frontside 451 ofsemiconductor material 401 to form first dopant region 408 betweenfrontside 451 and a location of the storage node 405. In the depictedexample, storage node 405 has already been formed; however, in otherexamples (like the example depicted in FIG. 3A) storage node 405 may nothave been formed.

FIG. 4B shows etching a trench from backside 453 of semiconductormaterial 401 into first dopant region 408. In this example, the implantdopant (e.g., nitrogen) damaged the crystal lattice of semiconductormaterial 401 between frontside 451 and the location of storage node 405.Etching the trench preferentially removes the crystal lattice that wasdamaged by the first dopant. This may produce a trench with asubstantially “T” shaped structure. Etching of semiconductor material401 may be accomplished by wet or dry etch depending on other processingconsiderations and the geometries formed. In the depicted example, thehorizontal component of the substantially “T” shaped structure is atleast partially laterally coextensive with storage node 405 to preventimage light from reaching storage node 405. In other examples thehorizontal component may have larger lateral bounds than the lateralbounds of storage node 405 or may even extend around the edges ofstorage node 405 to partially encircle storage node 405.

FIG. 4C shows backfilling the trench with at least one of a high-kmaterial or an oxide to form optical shield 407. In the depictedexample, a second dopant (e.g., boron) is implanted in semiconductormaterial 401 to compensate for inadvertent nitrogen doping in the regionof semiconductor material 401 that the storage node 405 is formed in.Moreover after etching the trench, the interior of the trench may beoxidized to form a liner (e.g., SiO₂). Once the liner is formed, a highk-oxide, metal, or semiconductor may be deposited in the trench. Asstated above, the various optical and electronic properties of thesematerials may be used to prevent light from reaching storage node 405.

Also depicted is forming additional pieces of device architecture suchas photodiode 403. In one example, forming the plurality of photodiodesincludes forming a region of photodiode 403 that is at least in partlaterally coextensive with an active region of transfer gate 411. Thusphotodiode 403 is positioned to absorb at least a portion of the imagelight directed toward the active region of transfer gate 411 before theportion of the image light reaches the active region. Moreover FIG. 4Cshows forming output gate 415 electrically coupled to storage gate 413to output the image charge from storage gate 413. FIG. 4C also showsimplanting floating diffusion 421 in frontside 451 of semiconductormaterial 401, and floating diffusion 421 is coupled to output gate 415to receive the image charge. Although not illustrated in FIG. 4C, anamplifier transistor may also be formed to be coupled to floatingdiffusion 421 to amplify the image charge in floating diffusion 421.

The above description of illustrated examples of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific examples of the invention are described herein forillustrative purposes, various modifications are possible within thescope of the invention, as those skilled in the relevant art willrecognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific examples disclosedin the specification. Rather, the scope of the invention is to bedetermined entirely by the following claims, which are to be construedin accordance with established doctrines of claim interpretation.

What is claimed is:
 1. A method of backside illuminated image sensorfabrication, comprising: forming a plurality of photodiodes in asemiconductor material, wherein the plurality of photodiodes aredisposed to receive image light through a backside of the backsideilluminated image sensor; forming a transfer gate coupled to extractimage charge from a photodiode in the plurality of photodiodes; andforming a storage gate coupled to the transfer gate to receive the imagecharge, wherein forming the storage gate includes: forming an opticalshield disposed within the semiconductor material; depositing a gateelectrode proximate to a frontside of the semiconductor material,wherein the frontside is opposite the backside; and implanting a storagenode in the semiconductor material, wherein the storage node is disposedin the semiconductor material between the optical shield and the gateelectrode, and wherein the storage node, the optical shield, and thegate electrode are all at least in part laterally coextensive with eachother.
 2. The method of claim 1, wherein forming the optical shieldincludes: implanting a first dopant from the frontside of thesemiconductor material to form a first dopant region between thebackside and a location of the storage node; etching a trench from thebackside of the semiconductor material into the first dopant region; andbackfilling the trench with at least one of a metal or an oxide.
 3. Themethod of claim 2, further comprising: implanting a second dopant withan opposite majority charge carrier type as the first dopant tocompensate for the first dopant; after etching the trench, oxidizing theinterior of the trench to form a liner; and depositing a high-k oxide inthe trench.
 4. The method of claim 3, wherein implanting the firstdopant includes implanting nitrogen to damage a crystal lattice of thesemiconductor material between the backside and the location of thestorage node, and wherein etching the trench preferentially removes thecrystal lattice that was damaged by the first dopant to produce a trenchwith a substantially “T” shaped structure.
 5. The method of claim 4,wherein a horizontal component of the substantially “T” shaped structureis at least partially laterally coextensive with the storage node. 6.The method of claim 1, wherein forming the optical shield in thesemiconductor material includes: implanting an impurity element betweenthe backside and a location of the storage node; and annealing theimpurity element.
 7. The method of claim 6, wherein implanting theimpurity element occurs through the frontside of the semiconductormaterial, and wherein the impurity element includes at least one ofgermanium or oxygen.
 8. The method of claim 7, wherein annealing theimpurity element forms the optical shield with at least one of SiGe orSiO₂, and wherein the optical shield is optically aligned with thestorage node and prevents image light from reaching the storage node. 9.The method of claim 1, wherein forming the plurality of photodiodesincludes forming a region of the photodiode that is at least in partlaterally coextensive with an active region of the transfer gate. 10.The method of claim 9, wherein the region of the photodiode ispositioned to absorb at least a portion of the image light directedtoward the active region of the transfer gate before the portion of theimage light reaches the active region.
 11. The method of claim 1,further comprising: forming an output gate electrically coupled to thestorage gate to output the image charge from the storage gate;implanting a floating diffusion in the frontside of the semiconductormaterial, wherein the floating diffusion is coupled to the output gateto receive the image charge; and forming an amplifier transistor coupledto the floating diffusion to amplify the image charge in the floatingdiffusion.